Guest's answer and explanation are correct; my approach would be:
Let J represent the number of CDs that Joel has.
Let M represent the number of CDs that Mariella has.
Let B represent the number of CDs that Blake has.
Since Joel has twice as many CD as Mariella has:
J = 2M (equation 1)
Since subtracting 7 from the number of CDs Joel has and dividing by 3 equals the number of CDs Blake has:
(J - 7) / 3 = B (equation 2)
Since Blake has 25 CDs, substitute 25 for B into equation (2):
(J - 7) / 3 = 25
---> Multiply both sides of this equation by 3:
J - 7 = 75
---> Add 7 to both sides of this equation:
J = 82
So, Joel has 82 CDs.
Now, substitute this value into equation 1:
82 = 2M
So, Mariella has 41 CDs.